Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state lighting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state lighting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
Solid state lighting panels are commonly used as backlights for small liquid crystal display (LCD) display screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting panels for general illumination, such as indoor lighting.
The color rendering index of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. For large-scale backlight and illumination applications, it is often desirable to provide a lighting source that generates white light having a high color rendering index, so that objects illuminated by the lighting panel may appear more natural. Accordingly, such lighting sources may typically include an array of solid state lighting devices including red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources. There are many different hues of light that may be considered “white.” For example, some “white” light, such as light generated by sodium vapor lighting devices, may appear yellowish in color, while other “white” light, such as light generated by some fluorescent lighting devices, may appear more bluish in color.
The chromaticity of a particular light source may be referred to as the “color point” of the source. For a white light source, the chromaticity may be referred to as the “white point” of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator heated to a given temperature. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the hue of the light source. White light typically has a CCT of between about 4000 and 8000K. White light with a CCT of 4000 has a yellowish color, while light with a CCT of 8000K is more bluish in color.
For larger illumination applications, multiple solid state lighting panels may be connected together, for example, in a two dimensional array, to form a lighting system. Unfortunately, however, the hue of white light generated by the lighting system may vary from panel to panel, and/or even from lighting device to lighting device. Such variations may result from a number of factors, including variations of intensity of emission from different LEDs, and/or variations in placement of LEDs in a lighting device and/or on a panel. Accordingly, in order to construct a multi-panel lighting system that produces a consistent hue of white light from panel to panel, it may be desirable to measure the hue and saturation, or chromaticity, of light generated by a large number of panels, and to select a subset of panels having a relatively close chromaticity for use in the multi-panel lighting system. This may result in decreased yields and/or increased inventory costs for a manufacturing process.
Moreover, even if a solid state lighting panel has a consistent, desired hue of light when it is first manufactured, the hue and/or brightness of solid state devices within the panel may vary non-uniformly over time and/or as a result of temperature variations, which may cause the overall color point of a lighting panel made up of the panels to change over time and/or may result in non-uniformity of color across the lighting panel. In addition, a user may wish to change the light output characteristics of a lighting panel in order to provide a desired hue and/or brightness level of the lighting panel.
Solid state lighting sources may have a number of advantages over conventional lighting sources for general illumination. For example, a conventional incandescent spotlight may include a 150 watt lamp projecting light from a 30 square inch aperture. Thus, the source may dissipate about 5 watts of power per square inch. Such sources may have an efficiency of no more than about 10 lumens per watt, which means that in terms of ability to generate light in a given area, such a source may generate about 50 lumens per square inch in a relatively small space.
A conventional incandescent spotlight provides a relatively bright, highly directed source of light. However, an incandescent spotlight may illuminate only a small area. Thus, even though an incandescent spot light has a relatively high light output, it may not be suitable for general illumination, for example illumination of a room. Thus, when used indoors, spotlights are typically reserved for accent or fill-in lighting applications.
Fluorescent light bulbs, on the other hand, produce light in a manner that is more suitable for general illumination. Fluorescent light bulbs approximate line sources of light, for which the intensity falls off in proportion to 1/r near the source, where r is the distance from the source. Furthermore, fluorescent light sources are typically grouped in a panel to approximate a plane source of light, which may be more useful for general interior illumination and/or other purposes, since the intensity of the light generated by a plane source may not drop off as quickly near the source as the intensity of a point or line source of light does.
A typical fluorescent light source may include three bulbs, which may emit 40 watts of light per bulb, for a total of 120 watts. A typical fluorescent panel for general interior illumination may have dimensions of about 24″×48″, which corresponds to about 0.1 watts per square inch of power dissipation. A fluorescent light source may produce up to about 65 lumens per watt, for an overall luminance of about 7 lumens per square inch for a 24″×48″ lighting panel. Even though this may be significantly less than the luminance of an incandescent spotlight of similar power, the distributed nature of a fluorescent light panel and its suitability for interior illumination has made fluorescent light panels a popular choice for general lighting applications. As noted above, however, fluorescent light may appear slightly bluish or washed out. Furthermore, fluorescent light bulbs may present environmental difficulties, since they may include mercury as a component.
Accordingly, there is a continued need for improved lighting sources for general illumination.